When the ground is just about frozen in my USDA Zone 5 garden, it's time to apply a winter mulch, especially around garden plants with shallow roots and any new additions that have not had a full season to get established.
The purpose of winter mulch is to keep the ground consistently frozen. Without this protection, the soil will likely thaw and refreeze. As the soil expands and contracts during this cycle of freezing and thawing, it can push plants up out of the ground, exposing their roots to biting cold and drying winds.
Plants with shallow root systems, like coral bells (Heuchera), foamflower (Tiarella) and foamy bells (Heucherella), are most apt to be dislodged by frost heaves, as the freeze-thaw cycle is known. Also susceptible are any perennials planted in the recent fall, because they haven't yet had time to develop an extensive root system. These are the plants to really focus on when it comes time to apply a winter mulch.
The best winter mulch for plants in cold zones is snow. Snow keeps the ground consistently frozen and, conveniently for the gardener, it disappears in the spring, allowing plants to gradually come out of dormancy. But increasingly we can't rely on a consistent snow cover, even in areas where that was once the norm.
Fallen deciduous leaves are one option for winter mulch, as is an additional layer of bark mulch. (Read advice on mulching with fallen leaves here.) But here's my favorite material to use instead: evergreen boughs.
Yes, I use evergreen boughs to protect my garden beds. We always gather greens from the farm where we get our Christmas tree. We’ve noticed that people cut down their tree and leave behind lower branches and other greens, so we sweep them up and bring them home (no charge). After the holidays, we cut the branches off our de-decorated tree and use those, too.
Related: "Ways to Repurpose a Christmas Tree"
Evergreen boughs are effective as winter mulch because they shield the soil from the sun, which is what causes thawing. And they have a few advantages over leaves or bark chips as winter mulching material:
First, boughs do not provide a haven for burrowing (and plant- or bulb-nibbling) voles and mice the way that leaves or shredded bark can.
Secondly, evergreen boughs are easy to gather up in early spring, when it's time for the plants to wake up. It's much easier to pick a loose layer of boughs up in early spring than to clear a layer of tree leaves from emerging perennials—without damaging the new growth on plants like heuchera or the early flowers of plants like hellebores.
Finally, a layer of leaves can become a soggy mess in wet winter weather, while evergreen boughs quickly shed water. This is especially key if you're trying to protect plants that are prone to rot in wet conditions.
Related: Now that you know about winter mulch, learn about using perennials themselves as mulch during the growing season. Read "Using Perennial Plants As a Living Green Mulch."
